a cpu option of the configure script as a method to specify a target cpu
* don't setup a cpu before scanning for --cpu=
* --cpu= option sets a 'cpu' variable, not a 'build_cpu', 'build_cpu' was not used anywhere.
* if cpu="" and ARCH != "" then cpu=$ARCH else cpu=`uname -m`
* replace "Build CPU" with "Target CPU" in the output of the configure script.
output this value only when not builing a cross compilers.
* remove a HOST_I386, ... defines from a config.h file.
thise defines are not used anywhere and cpu is now used to define a target cpu